1) As mentioned, you can use a dogbone adapter to go from 50 to 30. Though, most sites with 50amp service also have a 30amp plug. It's rare you find a 50amp only site.
2) Sanitize the system:
http://learntorv.com/eww-stinky-water-sanitizing-the-fresh-water-system/As for water in the water heater, open a hot water tap inside the camper to draw water in. Then open the overflow valve on the water heater itself periodically to see if it is full. Depending on winterization valves, don't trust that because you got water at the tap that the water heater is full; check it with the overflow valve.
3) Winterization:
http://learntorv.com/winterizing-your-campers-water-system/Otherwise, checkout my
Newbies, Get Started Here section of the site.